Xerath is a champion that requiere to always care about your positioning, because with your insane range its easy to loose view of your champion in a messy teamfight. But if you are too much near of the teamfight you will be focused and you range will be useless. In teamfights its key to burst down the enemy ad or apc! But the higher elos you get the harder this will be because they will position themselvs better so if you dont see the oppertunity to burst their carry down try to nuke the WHOLE enemy team and do as much burst as you can on as many people as you can ! After the first few seconds of a teamfights everybody on the enemy team should be around half to a quater hp try to chase them down ( but stay in range of your ulti of course !) and kill them with your ultimate. Theres nothing more to say gl hf on the fields of justice !

Pro

He has a GREAAAAAAAAT RAAAAANNGE on his spells
Xerath takes advantage many of the new system of calculation of magic penetration
He has a powerfull burst and can one shot a squishy man
He has a stunt to help for a gank or for flee
He can push very fast


Con:

No mobility on ultimate
Dependent on skillshots
Can't kill a tanks :/ (but can stunt it)

Passive:

This new passive give you mana each time you make a basic attack. It give you more mana if you make a basic attack on a champion than on a creep.

Q spell :

This is the main spell of Xerath. Try to use it on the most ennemies. You can also one shot a wave of minions when they are in align like these cats:
The Q spell range is a little bit more large than what is display, train with it and you will see...

It look like this in game:


W spell :

This spell is very usefull, it can slow yours ennemies so you can just use the W spell followed by the Q spell to deal a lot of damages.

It look like this in game:

E spell:

This spell can stunt, it's very usefull ! It add a little burst too but it's not the most important.

It look like this in game:

Ultimate (R spell):

The ultimate have on of the greatest range of the game and it deal a lot of damages.

Range:

The range is more large each level of the spell.

Range: 3200 / 4400 / 5600

Cost: 100 mana

Cooldown: 130 / 115 / 100 secondes

Damages per charge: 190 / 245 / 300 (+43% of your ap)

It give you magic resist against other midlaners because the majority are ap. It give you magic penetration because Xerath is AP x) and health to don't die when you are ganked. And ofc it give you ap.

This masteries give you more health and increase your magic resistance and your armor. It also give you a lot of ap and more damages. Personally I prefer place some points on the Defense tree than on the Utility tree (I test both)

This is my personal build. I take an doran's ring because it give you mana live and ap, best starter. You can also take the Crystalline Flask to get more sustain. Always pick the warding trinket.

I pick the Athene's Unholy Grail because too many good stats to pass up but against full ad comps a bit weaker, go morellos instead if you want. morellos give u more ap but no ap resist.

Up: R>Q>W>E
The Q is the main spell of course. The w add burst and the e help you to cast the combo.

OR

If u are the best caster ever, up R>W>Q>E. The W deal more damages if your ennemie is on the center (the damages are +91% of your AP if you cast in the center of the spell)

Xerath have to stunt the guy who is focused by the team with his E spell.